Precipitated calcium carbonates find use in a broad range of manufactured and consumer products ranging from paper and paper coatings to plastic and elastomer reinforcement to food supplements and cosmetics. Calcium carbonate is particularly useful in paper coatings, because it is relatively inexpensive and imparts good optical performance characteristics to paper resulting in a brighter, glossier more opaque paper, while also providing a high resistance to yellowing and aging.
PCC can be cleanly and conveniently produced in a precipitation reaction by reacting aqueous calcium hydroxide, Ca(OH)2 (also known as “milk of lime” or “MOL”) with carbon dioxide (“carbonation”) with water being generated as a by-product. Under certain reaction parameters, such as reaction temperature of 8° C. to 15° C., this precipitation reaction may produce “basic calcium carbonate” that, if desired, can be used as precursor for further conversion to forms of calcium carbonate, such as calcite, by further carbonation. Basic calcium carbonate is a desirable form of the material because it has a “platy” structure that is especially good at imparting desirable functional properties such as high gloss, whiteness and opacity when prepared as part of a paper coating.
It is often desired to produce precipitated calcium carbonate in specific forms and particle sizes such as the small particle size calcite form. Calcite has a trigonal crystalline form with crystal habits such as scalenohedral, rhombohedral, hexagonal prism, pinacoid, and cubic, and prismatic. These particular morphologies are important because the coating properties, such as light scattering, of a calcium carbonate material are strongly correlated to its morphology and particle size. Prismatic and rhombohedral-shaped PCC have maximum light scattering at 0.4 to 0.5 μm sized particles, while scalenohedral-shaped PCC has maximum light scattering of 0.9 to 1.5 μm particles. The aforementioned small-shaped sized, rhombohedral morphology makes the material particularly effective for use in a paper coating composition.
Given the forgoing there is a continuing need for calcium carbonate materials that impart excellent optical performance properties to paper when included in a paper coating composition. Such calcium carbonate materials should preferably be in a crystal form that is most likely to enhance such optical performance, and have other characteristics such as particle size and particle size distribution that further enhance the optical performance of paper.